The present invention relates to wideband signal transmission over a radio interface.
Usually, the transmit chain of a radio communication network comprises a power amplifier to amplify a signal before transmission over the radio interface.
On the one hand, the peak to average ratio (PAR) is an important characteristic of the signal submitted to the power amplifier. The peak to average ratio is defined by the ratio between the highest amplitude of the signal and the average amplitude of the signal. Wideband signals superposing information transmitted by several users using spread spectrum techniques (e.g. CDMA signals) are presenting a near-Gaussian frequency distribution with a stringent spectrum mask and wide envelope modulation dynamic resulting in high peak to average ratio (PAR about 10 dB).
On the other hand, a power amplifier is usually characterized by its efficiency defined as the ratio between the power of the signal at the amplifier output and the total power consumption of the power amplifier.
A signal having a high peak to average ratio results in a low efficiency of the power amplifier. For a standard compliant amplification of a CDMA signal with a PAR about 10 dB, an over-dimensioning of the amplifier chain of the same order is required (known as power amplifier backoff of about 10 dB). Such an over-dimensioning causes big penalties in terms of required energy supply or appropriate cooling means at the transmitters of the radio communication network (especially at the base stations). This over-dimensioning is nevertheless necessary not to reach the non-linear domain of the power amplifier characteristics. If this happens, peaks in the signal (called in the following overshots) would saturate the power amplifier causing high error rate at the receiver due to modulation distortion.
A known countermeasure to this problem is the clipping technique. It consists of eliminating, from the signal, the overshots having an amplitude above a threshold amplitude determined according to the characteristics of the power amplifier. This is usually obtained by saturating the signal at this amplitude threshold. The choice of the threshold value is a tradeoff between the power amplifier efficiency, the quality of the transmitted signal and the receiver sensitivity. Indeed, the lower the threshold is set, the more information contained in the overshots will be removed from the signal and the more efficient the receiver will have to be to recover information that have been eliminated because of clipping.
The usual clipping techniques have to face non-linear filtering problems. On the one hand, saturating the signal at the threshold value generates wide band noise which is not compatible with requirements set to the signal in term of spectrum mask. Indeed, the usual clipping techniques cause the signal spectrum to extend beyond the allowed spectrum mask, causing interference in adjacent frequency channels. On the other hand, when filtering is performed to eliminate the noise outside the allowed spectrum mask, the memory effect intrinsic to filtering regenerates signal overshots above the desired threshold value.
These drawbacks are even harder to cope with when the power amplifier is shared between several adjacent wideband carriers to optimize cost and size of the transmitter. In this case, the signal provided to the amplifier is the sum of several frequency multiplexed signals, which tend to have a Raleigh frequency distribution again even if each individual signal has been clipped by the usual known method. A Raleigh frequency distribution shows the same drawbacks in term of overshots as a Gaussian distribution.
Moreover, an acceptable power amplifier efficiency is only provided at the price of tremendous processing efforts not compatible with usual computing capabilities.
An aspect of the present invention is to provide a clipping method that fulfills the requirements in terms of frequency spectrum range of the clipped signal with a reasonable processing effort.
Another aspect of the present invention is to provide a transmitter implementing this method.